We have undertaken the synthesis of several 3-glycosyl-5-aminopyrazolo(4,3-d)-pyrimidine-7-thiones as analogs of thioguanine nucleosides that show clinically effective anticancer properties. These target componds will be the first C-numleosides of the thioguanine type, that have the inherent advantage of resistance to nucleoside cleavage. The synthetic approach is adapted from our earlier research on synthesis of formycin B (3-ribosyl-pyrazolo(4,3-d)pyrimidine-7-one). Sugars to be incorporated as the 3-glycosyl substituent include beta-D-ribo, 2-deoxy-alpha- and beta-D-ribo, and beta-D-arabino. As a result of anomerization that appears to occur in some of the synthetic steps, the alpha-D-ribo and alpha-D-arabino isomers may also become available for comparison. Of secondary importance, precursors to the above 5-amino-7-thiones will be the corresponding 5-amino-7-ones, which will be the first guanine-type C-nucleosides in the formycin series.